Gene Expression: From DNA to Protein

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Gene Expression

Central Dogma of Molecular Biology

The central dogma of molecular biology describes the unidirectional flow of genetic information from DNA to RNA to protein. This process is fundamental to all living organisms and explains how genetic information is expressed as functional products.

Transcription: The process of synthesizing RNA from a DNA template.
Translation: The process of synthesizing proteins using the information encoded in mRNA.
Gene Expression: The full process by which genotype becomes expressed as phenotype, encompassing both transcription and translation.

Central Dogma: DNA to RNA to Protein

Transcription

Introduction to Transcription

Transcription is the process by which an RNA molecule is synthesized from a DNA template within a gene. Genes are specific sequences of DNA that encode products such as proteins or functional RNAs.

Promoter: DNA sequence where transcription begins; site of RNA polymerase attachment.
Terminator: DNA sequence where transcription ends.
RNA Polymerase: Enzyme that synthesizes RNA from DNA without the need for a primer.
Upstream/Downstream: Terms describing directionality relative to the transcription start site.

Gene structure: promoter, coding sequence, terminator

DNA Strands in Transcription

Genes are located on double-stranded DNA, but only one strand (the template strand) is used for RNA synthesis. The other strand is called the coding strand and has the same sequence as the RNA (except T is replaced by U).

Template Strand: Used by RNA polymerase to build the RNA transcript.
Coding Strand: Has the same sequence as the RNA (except T/U difference).
Base Pairing: A pairs with U (in RNA), C with G.

Transcription: DNA to RNA sequence

Steps of Transcription

Transcription occurs in three main steps: initiation, elongation, and termination.

Initiation: RNA polymerase binds to the promoter and unwinds the DNA.
Elongation: RNA polymerase synthesizes the RNA strand by adding nucleotides in the 5' to 3' direction.
Termination: RNA polymerase reaches the terminator sequence and releases the RNA transcript.

Initiation: Prokaryotes vs. Eukaryotes Elongation of Transcription Termination of Transcription

Eukaryotic RNA Processing & Splicing

In eukaryotes, the initial RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA ready for translation.

5' Cap: Modified guanine nucleotide added to the 5' end.
Poly-A Tail: String of adenine nucleotides added to the 3' end.
Functions: Facilitate export from nucleus, protect from degradation, help ribosome binding.

Pre-mRNA to Modified mRNA RNA Processing: 5' cap and Poly-A tail

RNA Splicing

Genes in eukaryotes contain coding (exons) and noncoding (introns) regions. Splicing removes introns and joins exons to form mature mRNA. Alternative splicing allows a single gene to code for multiple proteins.

Introns: Noncoding regions removed from pre-mRNA.
Exons: Coding regions joined together in mature mRNA.
Spliceosome: Complex responsible for splicing.
Alternative Splicing: Generates different mRNA products from the same gene.

Splicing and alternative splicing

Types of RNA

Major Types of RNA

Cells use several types of RNA, each with a distinct function in gene expression.

Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes; contains codons.
Ribosomal RNA (rRNA): Structural and catalytic component of ribosomes.
Transfer RNA (tRNA): Brings amino acids to the ribosome; contains anticodons complementary to mRNA codons.

Types of RNA: mRNA, rRNA, tRNA

The Genetic Code

Codons and the Genetic Code

The genetic code is a set of rules by which information encoded in mRNA is translated into proteins. Each codon (three nucleotides) specifies a particular amino acid.

Redundancy: Multiple codons can code for the same amino acid.
Start Codon: AUG (Methionine) signals the start of translation.
Stop Codons: UAA, UAG, UGA signal termination of translation.

Genetic code table DNA to mRNA to polypeptide

Translation

Introduction to Translation

Translation is the process by which ribosomes synthesize proteins using the sequence of codons in mRNA. tRNA molecules bring amino acids to the ribosome, matching their anticodons to mRNA codons.

Ribosome: Site of protein synthesis, composed of rRNA and proteins.
tRNA: Carries specific amino acids and matches them to mRNA codons via its anticodon.
Charged tRNA: tRNA attached to an amino acid.
Discharged tRNA: tRNA without an amino acid.

Translation: tRNA and ribosome tRNA structure

Ribosome Structure and tRNA Binding Sites

Ribosomes have two subunits and three tRNA binding sites: A (aminoacyl), P (peptidyl), and E (exit).

Prokaryotic Ribosome: 70S (50S + 30S subunits).
Eukaryotic Ribosome: 80S (60S + 40S subunits).
A-site: Holds tRNA with the next amino acid.
P-site: Holds tRNA with the growing polypeptide chain.
E-site: Site where discharged tRNAs exit.

Ribosome subunits and tRNA binding sites tRNA binding sites: A, P, E

Steps of Translation

Translation occurs in three main steps: initiation, elongation, and termination.

Initiation: Small ribosomal subunit binds mRNA and initiator tRNA (carrying methionine), followed by the large subunit.
Elongation: Amino acids are added one by one to the growing chain; ribosome moves along mRNA 5' to 3'.
Termination: Stop codon is reached, release factor binds, and the polypeptide is released.

Initiation of translation Elongation of translation Termination of translation

Post-Translational Modification

Types and Functions

After translation, proteins may undergo covalent modifications that regulate their activity, stability, or localization. These are called post-translational modifications (PTMs).

Methylation
Acetylation
Ubiquitination
Phosphorylation
Glycosylation: Addition of carbohydrates to proteins.

Post-translational modifications

Comparison: Transcription vs. Translation

Transcription and translation are distinct but related processes in gene expression.

	Transcription	Translation
Product Formed	RNA Molecule	Polypeptide (Protein)
Macromolecule Change?	Nucleic Acid → Nucleic Acid	Nucleic Acid → Protein
Major Enzyme/Structure	RNA Polymerase	Ribosome
Location	Nucleus (Eukaryotes)	Cytoplasm
Direction of Synthesis	5' → 3'	N-terminus to C-terminus

Transcription vs. Translation comparison table

Mutations

Types and Effects

Mutations are permanent changes in the DNA sequence. They can affect gene expression and protein function, and may be harmful, beneficial, or neutral. Mutations can occur spontaneously or be induced by environmental factors (mutagens).

Point Mutation: Change in a single nucleotide.
Silent Mutation: No change in amino acid sequence.
Missense Mutation: Changes one amino acid in the protein.
Nonsense Mutation: Introduces a premature stop codon.
Frameshift Mutation: Insertion or deletion of nucleotides that alters the reading frame.

Types of mutations

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